1. Field of the Invention
The present invention relates to a semiconductor memory device, and relates, for example, to a FBC (Floating Body Cell) memory that stores information by accumulating majority carriers in a floating body of a field-effect transistor (FET).
1. Related Art
In recent years, there is an FBC memory device as a semiconductor memory device expected as a memory that replaces a DRAM. The FBC memory device has an FET equipped with a floating body (hereinafter, also “body area”) on an SOI (Silicon On Insulator) substrate, and stores data “1” or data “0” depending on the number of majority carriers accumulated in this body area. For example, a memory cell MC is an N-type NET, and a state that the number of holes within the body area is large is set as data “1” and a state that the number of holes is small is set as data “0”. The FBC memory device is superior to an 1T (Transistor)-1C (Capacitor)-type DRAM in that the FBC memory device can be made fine and can increase a memory capacity.
The FBC memory device generates a reference current between a current flowing “1” cell and a current flowing “0” cell, and compares this reference current with the current to identify whether this data is “1” or “0”. As a system that generates a reference current, a dummy cell system is available. The dummy cell system generates an intermediate current between a current flowing the memory cell storing data “1” and a current flowing the memory cell storing data “0” as a reference current from a pair of a dummy cell storing data “1” and a dummy cell storing data “0”.
The dummy cell system has required a writing circuit exclusive for dummy cells, to write data into dummy cells and to refresh dummy cells. Therefore, a size of a memory area becomes large, and it is difficult to further decrease the size of the FBC memory device.